Abstract <p>A low-cost time-resolved planar quantitative light sheet system was designed to visualize the transported scalar concentrations of short cylindrical hole-jets under oscillating mainstream. Combined influences of length-to-diameter ratio of cylindrical hole, mainstream oscillating frequency and cooling air flowrate on time-averaged film effectiveness and film cooling instability were discussed. Film cooling mechanisms were revealed through extracting the spatial and temporal gradients of concentrations, as well as the hidden coherent flow structures. The experimental results revealed that mainstream oscillations enhance the near-wall motion of shear layer large-scale coherent flow structure, essentially reducing the film effectiveness and increasing the cooling unsteadiness. Regardless of whether the mainstream is oscillating or not, in the range of length-to-diameter ratio from 1.5 to 5, the jet with 5 have the largest benefit on the wall thermal protection and the jet with 2.5 can generate the highest-level cooling unsteadiness. Mainstream oscillation changes some original trends in film cooling under stable mainstream condition, inducing the lower film effectiveness at high blowing ratio as well as the larger sensitivity of film effectiveness to blowing ratio for the shorter hole. </p> Graphical abstract <p></p>